The present invention relates to a new and improved construction of an endoprosthesis for a hip-joint socket which is of the type comprising a shell or jacket sleeve intended to receive a bearing or support socket. The shell or jacket sleeve possesses the shape of a body of revolution or rotation which tapers or narrows towards a pole or apex region and which carries a self-tapping or self-cutting thread or threading at its outer surface.
In this respect, it is to be noted that also in the context of the present development the expression "self-tapping thread" or "self-cutting thread" designates a thread or threading wherein the thread flight or course for each revolution or convolution is subdivided into sections or portions by a plurality of gaps or openings, and the trailing wall of each gap, as viewed in the threading-in direction, forms a cutting surface or cutter, the cutter shape of which is governed by the thread profile or sectional shape.
Endoprostheses of the previously mentioned type are known in this technology. The shape of the shell or jacket sleeve results in the fact that also the self-tapping thread can be compared to a certain extent to a conical external thread. With the state-of-the-art endoprostheses, the self-tapping thread possesses the profile or sectional shape of approximately a pointed thread or a trapezoidal thread, and the thread pitch is approximately constant at all thread sections or portions.
Since, as stated, the thread or threading can be compared to a conical thread and with the heretofore constructions of endoprostheses the thread pitch is constant at all thread sections, during threading-in of the shell sleeve into the substance of the bone, the sections of the thread which follow the cutters exert a pressure upon the bone substance both in radial direction and in axial direction in a manner such as if at the neck or entrance opening of the so-to-speak female thread which has been cut into the bone substance, there were driven-in wedges in the circumferential direction. As a result, for the threading-in of the shell sleeve, there must be applied a comparatively high torque or rotational moment. Hence, the operating surgeon does not always sense the point in time when the shell sleeve has been threaded-in to a satisfactory depth. Moreover, with the heretofore known constructions of the prior art endoprostheses, there cannot be totally eliminated the risk that a fissure or crack will form in the bone substance or material due to the aforementioned prevailing pressure. Furthermore, this bone fissure need not necessarily occur during the implantation, rather, and this is even worse, when following the surgical operation the implanted hip-joint is again subjected to normal loads. An incompletely threaded-in sleeve and/or the formation of fissures in the bone substance result in a loose seating of the implanted hip-joint socket and possibly can result in the detrimental consequence that there is required a renewed surgical operation.